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Extract a given number of randomly selected elements from a list

One of the problems from https://ocaml.org/learn/tutorials/99problems.html

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open System

let extract list num = 
    let rec pick list num i = 
        match list with
        | h :: t -> 
            if i = num then h
            else pick t num (i + 1)
        | _ -> failwith "error"
    
    let getRandom = 
        let r = ref (Random())
        fun max -> (!r).Next(1, max + 1)
    
    let rec extractInner list num i = 
        if i = num then []
        else (pick list (getRandom list.Length) 1) :: extractInner list num (i + 1)
    
    extractInner list num 0
namespace System
val extract : list:'a list -> num:int -> 'a list

Full name: Script.extract
Multiple items
val list : 'a list

--------------------
type 'T list = List<'T>

Full name: Microsoft.FSharp.Collections.list<_>
val num : int
val pick : ('b list -> int -> int -> 'b)
Multiple items
val list : 'b list

--------------------
type 'T list = List<'T>

Full name: Microsoft.FSharp.Collections.list<_>
val i : int
val h : 'b
val t : 'b list
val failwith : message:string -> 'T

Full name: Microsoft.FSharp.Core.Operators.failwith
val getRandom : (int -> int)
val r : Random ref
Multiple items
val ref : value:'T -> 'T ref

Full name: Microsoft.FSharp.Core.Operators.ref

--------------------
type 'T ref = Ref<'T>

Full name: Microsoft.FSharp.Core.ref<_>
Multiple items
type Random =
  new : unit -> Random + 1 overload
  member Next : unit -> int + 2 overloads
  member NextBytes : buffer:byte[] -> unit
  member NextDouble : unit -> float

Full name: System.Random

--------------------
Random() : unit
Random(Seed: int) : unit
val max : int
val extractInner : ('b list -> int -> int -> 'b list)
property List.Length: int
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More information

Link:http://fssnip.net/ro
Posted:9 years ago
Author:Vladimir Khorikov
Tags: ocaml , f#

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